1.Field of the Invention
The present invention relates to the art of controlling the emissions of acid gas, particularly sulphur oxides (SO.sub.2 and SO.sub.3) from flue gases produced in a coal fired boiler. Sulphur oxides are formed when coal containing sulphur is burned in a furnace. The combustion process results in the production of hot flue gas which contains unacceptable levels of SO.sub.2 and/or SO.sub.3. These sulphur oxides are generally eliminated or reduced to acceptable levels by scrubbing the flue gas with particulate scrubbing materials such as limestone granules. Typically, the flue gas containing sulphur oxide is scrubbed by passing the flue gas through a bed of wet limestone which reacts with the sulphur oxide contained in the flue gas to produce a solid reaction product (calcium sulphate and calcium sulphite). The production of a solid reaction product containing sulphur effectively removes the gaseous sulphur compounds from the flue gas.
The solid reaction product forms a crust around the limestone particles which must be periodically removed because the presence of the crest produces a surface blinding effect which interferes with the reaction between the scrubbing particles and the sulphur oxides. Thus, the particles containing a crust of reaction product are periodically regenerated by removing the crust so as to expose a fresh surface of scrubbing material for reaction with the sulphur oxides.
The present invention relates to an improvement in the configuration of a moving bed of wet scrubbing material which permits the scrubbing to proceed in a highly effective and efficient manner.
2. Description of the Prior Art
The prior art generally relies upon the use of a bed of wet limestone particles for scrubbing the sulphur oxide from the flue gas. Generally, fixed beds are used such as described in U.S. Pat. Nos. 4,663,136 and 4,764,348 (the specifications of which are incorporated herein by reference). However, such fixed beds cannot be operated continuously because of the need to periodically regenerate the limestone by removing the crust of calcium sulphate/sulphite therefrom so as to avoid the surface blinding effect. Such regeneration requires agitating the encrusted limestone particles to break off the crest of reaction product. Once the regeneration step has been completed, the bed can once again be used to scrub the sulphur oxide from the flue gas.
FIG. 1 shows a simplified flow schematic of a prior art fixed bed unit. In the unit shown in FIG. 1, flue gas enters a spray chamber 1 where it is conditioned to the desired temperature and humidity. Conditioning is achieved by adding water via an atomizing nozzle 2 and/or by injecting live steam into the chamber by means of injector 3. The conditioned flue gas enters a reactor 4 which contains the fixed bed of limestone which is preferably wet with just enough water so that the particles are coated with a film of water. The fixed bed of limestone may comprise a removable basket filled with limestone. The limestone may be wet by spraying with overhead sprayer 5. The flue gas passes downward through the fixed bed of limestone. Sulphur dioxide is absorbed from the flue gas into the water film where it subsequently reacts with dissolved limestone. It is this reaction which is believed to produce the crust of calcium sulphate and sulphite. The presence of a liquid water film on the limestone is critical since dry limestone does not readily react with sulphur oxides in the flue gas. The scrubbed flue gases are drawn out from the bottom of the limestone bed by an induced draft (ID) fan 6 and are returned to duct 7 leading to a stack for elimination into the atmosphere.
It is known to use a moving bed of limestone particles in the prior an for various purposes. One such apparatus is described in U.S. Pat. No. 3,976,746 (Shale et al.) However, Shale reports that the crest of reaction product must be periodically removed in order to regenerate the limestone for further reaction. The periodic removal, according to Shale, can be accomplished by mechanical agitation. In order to accomplish the required periodic regeneration of the limestone, Shale et al. provides a mechanical device such as a vibrating screen to remove the reaction crust from the limestone particles.
Whenever flue gas flows at a high velocity through a moving bed of scrubbing particles, such as the moving bed described by Shale et al., there is a tendency for the particles, especially the smaller particles, to be carried over and thereby removed from the bed due to the force of the flowing gas passing through the bed. The carry-over of the scrubbing particles results in an undesirable loss of limestone from the bed. Furthermore, the scrubbing particles which are lost due to "carry-over" will become entrained in the flow of flue gas unless special precautions are taken. Therefore, these carry-over particles will be easily drawn into the ID fan which is typically present for assisting in the flow of gas through the system.
Shale et al. fails to provide any mechanical means for reducing the above-described carry-over of scrubbing particles. Thus if the operators of a SO.sub.2 scrubbing device wanted to minimize the carry-over, they would have to install a screen or filter or reduce the mount of flow gas through the bed. Both of these approaches are undesirable. A filter or screen would become clogged over a period of time and produce an undesirable pressure drop across the bed. Reducing the gas flow would produce a commensurate reduction of efficiency.
It is known in the art to contain a moving bed of particles in a device having a series of louvers on the gas inlet and gas outlet side of the bed. The louvers may be designed with sufficient overlap to help contain the particles in the bed while allowing the gas to pass freely through the bed. FIG. 2 illustrates the basic configuration for a typical louvered moving bed.
Moving beds having louvers are disclosed, for example, in U.S. Pat. Nos. 4,670,226 (Furuyama et al.); 5,160,708 (Kodama et al.); 4,254,557 (Mayer et al.) and 4,254,616 (Siminski et al.)
The use of louvered moving beds per se without any modification have been shown to be inadequate in limestone emission control devices. For example, in early experimental work which lead to the present invention, it was determined that a LEC (limestone emission control) moving bed pilot plant which relied only on the louvers to retain the limestone in the bed did not accomplish this purpose. During operation, limestone from the bed was drawn in the outlet plenum and subsequently into the ID fan. In order to prevent this limestone loss from the bed and to protect the ID fan, a screen was attached to the inside (limestone bed side) of the outlet louvers in order to keep the limestone in the bed. However, the presence of the screen introduced another operational problem. In particular, damp fines were deposited on the screen which resulted in plugging and a significant increase in the pressure drop across the limestone bed. This plugging (i.e., blinding) of the screen occurred most rapidly at the top of the bed which is normally the most reactive part of the bed. As a result of the screen blinding at the top of the bed, the flue gas would begin to flow preferentially through the bottom portions of the bed which are normally the least active part of the bed. This action would result in a simultaneous increase in pressure drop across the bed, a decrease in SO.sub.2 removal performance and a decrease in the flue gas flow rate.
Attempts have been made to minimize the carry-over problem by adjusting the size of the limestone particles. However, larger particles which may be sufficiently heavy to eliminate the carry-over problem are not as efficient as the smaller particles in their ability to remove sulphur oxides from the flue gas. Combinations of large and small particles have been observed to combine the worst features of both.
Attempts have also been made to overcome the carry-over problem by adjusting the design of the louvers. It would be expected that larger louvers with sufficient overlap would be adequate to avoid carry-over because louvered dry scrubbers containing similarly sized limestone are said to be useful for removing particles from a gas stream. However, it has been determined that using louvers sloped at 70.degree. C. which are 11" long with a 31/2" overlap failed to prevent carry-over.
Obviously, it would be advantageous to have the capability of scrubbing large quantifies of gas in a short period of time. Thus, it is desirable to operate limestone emission control systems with a high flow rate of flue gas through the moving bed in order to attain this capability. However, higher flow rates are associated with a greater degree of particle carry-over especially when smaller more efficient sized particles are used. It would be desirable to be able to use smaller particles without the problems associated with carry-over because smaller particles have a higher surface area to volume ratio, and thus they will provide a more efficient scrubbing capability. As a result of this greater scrubbing capability, the system can effectively scrub flue gas having a high sulphur oxide content.
All of the attempts to eliminate the carry-over of limestone in an efficient moving bed limestone emission control system have proven to be inadequate. Thus, there exists a long felt need in the art to provide a solution to this problem without sacrificing the sulphur oxide scrubbing efficiency of a moving bed limestone emission control device.